Conventional test devices require signal profiles to be calibrated with regard to their levels and time or phase relationships. Signal propagation times of test signals deviate both within a test device and during a signal transmission from the test device to the wafers to be tested, which may contain semiconductor components to be tested.
Furthermore, during the testing of wafers to be tested, one problem is that the different signal propagation times of signals from the component to be tested (wafer to be tested) back to the test device must likewise be compensated for. Calibration of test devices becomes absolutely necessary in particular when the deviations caused by signal propagation times are of the same order of magnitude as a typical cycle time of the components to be tested on a wafer to be tested, since otherwise correct testing of components to be tested becomes impossible.
In conventional test devices variable delay elements are arranged for the compensation of different signal propagation times, which elements are set during a calibration of a test device in such a way that propagation time differences are minimized or eliminated.
FIG. 4 diagrammatically shows a flow diagram for the testing of a wafer to be tested in a conventional test device. A connection of peripheral units, such as, for example, suitable measuring instruments (oscilloscope, voltmeter and ammeter, propagation time difference, time difference and phase difference measuring units, etc.), is required in a conventional manner.
A test or calibration sequence begins at a step S401.
In a step S402, firstly the abovementioned peripheral units are connected in order to be able to carry out corresponding propagation time measurements, phase difference measurements, time difference measurements, etc.
Essentially two different methods are used in a conventional manner:
(i) A Reflection Measurement (Time Domain Reflectometry, TDR):
In this case, firstly driver units of the test device are measured, i.e. a signal path from the test device to a wafer to be tested or to a component to be tested. For this purpose, contacts at an interface of the component to be tested are either left unconnected or short-circuited to ground.
It follows from this that the signals proceeding from driver units are reflected (reflection measurement) at the interface of the module to be tested. A temporal difference (propagation time difference, phase difference, time difference) between the originally emitted signals and the reflected signals is determined by a suitable measuring device, whereupon compensation of existing propagation time differences is made possible.
A significant disadvantage of the TDR method is that a reflected signal is generally disturbed or impaired by discontinuities of a characteristic impedance on the signal path, this disturbance being caused in particular by the fact that the reflected signal disadvantageously has to pass through the signal path twice.
During an automatic determination of a temporal position of the reflected signal with regard to a reference signal, this results in an unacceptable error. Further errors occur in the conventional method for calibrating a test device by virtue of the fact that an ideal short circuit (connection to ground) or an ideal open circuit (unconnected state) cannot be achieved at an interface of a module to be tested, as a result of which a signal waveform is subject to modifications.
A further disadvantage of the TDR method is that, for each individual measurement, it is necessary to wait for a complete go and return passage of the signal from the test device to the component to be tested (the wafer to be tested) and back from there, in order to carry out a time difference measurement, for example. This inexpediently restricts a maximum repetition rate and a practically achievable statistical stability of the measurement results.
It is furthermore disadvantageous that the TDR method can be parallelized only to a very limited extent, so that a calibration time rises with a rising number of signal paths.
(ii) Method Using an External Measuring Instrument (e.g. an Oscilloscope)
In this case, one or more input connections of the measuring instrument are connected via a suitable device to each individual output connection of an interface of the module to be tested, for which purpose a robot arm or a relay matrix has to be provided in the case of automatic test devices. In this case, it is particularly disadvantageous that a calibration requires a very long time, since individual signal paths (channels) have to be measured sequentially. Together with considerable setup times when using external calibration equipment, this method is highly inefficient.
After a calibration effected in a step S403, the processing proceeds to a step S404. In step S404, a wafer to be tested is introduced into the test device and subsequently tested in a step S405.
The test result, which comprises, in particular, propagation time differences in signal paths to be tested, is evaluated in a step S406. In a step S407, an interrogation is effected to determine whether or not predeterminable criteria are met in respect of a quality of a calibration of the test device. A calibration quality can be determined using measurement results of a multiplicity of wafers. A decision as to whether a renewed calibration is necessary arises, for example, from a calibration validity duration prescribed by the manufacturer of the test device, or from considerations of the test results of wafers to be tested. If it is then determined that a calibration is necessary, the processing proceeds to step S402, in order to run through a renewed calibration mode. If the criteria entered into step S407 are met, then the processing proceeds to a step S408, in which an interrogation is effected to determine whether a next wafer is to be tested.
If it is ascertained in step S408 that a next wafer is to be tested, then the processing proceeds to a step S409, in which a new wafer to be tested is provided. The new wafer to be tested which is provided in step S409 is introduced into the test sequence of the test device in step S404.
If it is ascertained in step S408 that no further wafers are to be tested, then the processing proceeds to a step S410, in which the test sequence in the test device is ended.
It is a particular disadvantage of conventional methods for testing wafers to be tested that between a calibration sequence and a test sequence there are considerable differences in respect of the test sequence, the test environment, the test geometry, etc.
In a disadvantageous manner, it is not possible to integrate a calibration sequence with sufficient accuracy in an automatic test sequence in a test device, since, on the one hand, extensive reflection measurements (described above under (i)) have to be carried out, or a considerable outlay is necessary in the case of a connection of peripheral units (described above under point (ii) and with reference to FIG. 4). In order to provide a calibration with a sufficient accuracy, manual procedure has to be effected in the case of calibration methods according to the prior art.